The subject matter described herein relates generally to devices and methods for the measurement of a specific ion activity or concentration in solution based on potentiometric sensors (potential difference or voltage based measuring). An example of such measuring is use of a pH meter to determine the pH of a solution.
In a potentiometric measuring system, meter circuitry is designed to use readings of relative voltage, as sensed through measuring and reference electrodes, to calculate an ion concentration (hydrogen ion concentration in the case of pH metering). Typically, a voltage is sensed by an electrode measuring half cell and a voltage is sensed by a reference half cell that are electrically connected to a suitable measuring meter (meter circuitry). It is well known that hydrogen ion concentration in a given sample solution can be measured with such a pH measuring system.
In a conventional set up, reference electrodes (for example Ag/AgCl electrodes) are used together with measuring electrodes in a complete cell. These reference electrodes and measuring electrodes may be used in concert to determine ion concentration in a sample (for example, pH or another ion). The reference electrode is designed in order to maintain its potential as constant as possible throughout the measuring process. In contrast, the measuring electrode is designed such that its potential is a function of the concentration of the ion being tested. Constancy of the reference electrode's potential has been achieved by the presence of a saturated electrolyte salt bridge or junction. The reference electrolyte leaks slowly through the junction. The relative potential difference between the reference and measuring electrodes may be used to calculate the concentration of the ion in the sample, and may be displayed on a millivolt (mV) instrument (potentiometer).
The potential of a complete cell may be represented by:ECell=Emeas−(Eref+Ej)where Emeas, Eref, and Ej are the potentials of the measuring electrode, the reference electrode and the junction.
The electrode half cells (reference and measuring) are electrically connected to a pH meter circuitry to produce measurements in millivolts. The millivolt readings can be used to represent hydrogen ion activity in the solution being measured. Such conventional systems and the parts thereof are well known to those skilled in the art and are available in the commercial marketplace from various manufacturers such as Hach Company of Loveland, Colo. Thus, when the description set forth herein references examples referring to conventional components such as pH glass, it is intended to mean a conventional component such as pH glass of the type which is sold in the commercial marketplace by Hach Company of Loveland, Colo.